(core) hide long sequences of unchanged rows in diffs

Summary:
It can be hard to find changes, even when highlighted, in a table with many rows.  This diff replaces long sequences of unchanged rows with a row containing "..."s.

With daff, I found that it is important to do this for sequences of unchanged columns also, but not tackling that yet.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2666

app/plugin/objtypes.ts

@@ -100,6 +100,15 @@ export class PendingValue {
   }
 }
 
+/**
+ * A trivial placeholder for a value that won't be shown.
+ */
+export class SkipValue {
+  public toString() {
+    return '...';
+  }
+}
+
 /**
  * Produces a Grist-encoded version of the value, e.g. turning a Date into ['d', timestamp].
  * Returns ['U', repr(value)] if it fails to encode otherwise.
@@ -162,6 +171,7 @@ export function decodeObject(value: CellValue): unknown {
       case 'O': return mapValues(args[0] as {[key: string]: CellValue}, decodeObject, {sort: true});
       case 'P': return new PendingValue();
       case 'R': return new Reference(String(args[0]), args[1]);
+      case 'S': return new SkipValue();
       case 'U': return new UnknownValue(args[0]);
     }
   } catch (e) {